Great Circle tidal streams evidence for a nearly spherical m

An all high-latitude sky survey for cool carbon giant stars in the Galactic halo has revealed 75 such stars, of which the majority are new detections. Of these, more than half are clustered on a Great Circle on the sky which intersects the center of Sagitt

GreatCircletidalstreams:

evidenceforanearlysphericalmassivedarkhaloaroundthe

MilkyWay

RodrigoIbata1,GeraintF.Lewis2,3,4,MichaelIrwin5,EdwardTotten6,ThomasQuinn3

arXiv:astro-ph/0004011v2 7 Nov 2000

ABSTRACT

Anallhigh-latitudeskysurveyforcoolcarbongiantstarsintheGalactichalohasrevealed75suchstars,ofwhichthemajorityarenewdetections.Ofthese,morethanhalfareclusteredonaGreatCircleontheskywhichintersectsthecenterofSagittariusdwarfgalaxyandisparalleltoitspropermotionvector,whilemanyoftheremainderareoutlyingMagellanicCloudcarbonstars.PreviousnumericalexperimentsofthedisruptionoftheSagittariusdwarfgalaxy(theclosestoftheGalacticsatellitegalaxies)predictedthatthee ectofthestrongtides,duringitsrepeatedcloseencounterswiththeMilkyWay,wouldbetoslowlydisruptthatgalaxy.Duetothesmallvelocitydispersionofthedisruptedparticles,thesedisperseslowlyalong(approximately)theorbitalpathoftheprogenitor,eventuallygivingrisetoaverylongstreamoftidaldebrissurroundingourGalaxy.Themorerecentlydisruptedfragmentsofthisstreamshouldcontainasimilarmixofstellarpopulationstothatfoundintheprogenitor,whichincludesgiantcarbonstars.

GiventhemeasuredpositionandvelocityoftheSagittariusdwarf,we rstintegrateitsorbitassumingastandardsphericalmodelfortheGalacticpotential,and ndboththatthepathoftheorbitintersectsthepositionofthestream,andthattheradialvelocityoftheorbit,asviewedfromtheSolarposition,agreesverywellwiththeobservedradialvelocitiesofthecarbonstars.Wealsopresentapole-countanalysisofthecarbonstardistribution,whichclearlyindicatesthattheGreatCirclestreamwehaveisolatedisstatisticallysigni cant,beinga5-6σover-density.ThesetwoargumentsstronglysupportourconclusionthatalargefractionoftheHalocarbonstarsoriginatedintheSagittariusdwarfgalaxy.

ThestreamorbitstheGalaxybetweenthepresentlocationoftheSagittariusdwarf,16kpcfromtheGalacticcenter,andthemostdistantstreamcarbonstar,at～60kpc.ItfollowsneitherapolarnoraGalacticplaneorbit,sothatalargerangeinbothGalacticRandzdistancesareprobed.ThatthestreamisobservedasaGreatCircleindicatesthattheGalaxydoesnotexertasigni canttorqueuponthestream,sotheGalacticpotentialmustbenearlysphericalintheregionsprobedbythestream.Furthermore,theradialmassdistributionoftheHalomustallowaparticleatthepositionandwiththevelocityoftheSagittariusdwarfgalaxytoreachthedistanceofthefurtheststreamcarbonstars.Thus,theSagittariusdwarfgalaxytidalstreamgivesaverypowerfulmeanstoconstrainthemassdistributionitresidesin,thatis,thedarkhalo.

WepresentN-bodyexperimentssimulatingthisdisruptionprocessasafunctionofthedis-tributionofmassintheGalactichalo.Alikelihoodanalysisshowsthat,intheGalactocentricdistancerange16kpc<R<60kpc,thedarkhaloismostlikelyalmostspherical.Weruleout,athighcon dencelevels,thepossibilitythattheHaloissigni cantlyoblate,withisodensitycon-toursofaspectqm<0.7.Thisresultisquiteunexpectedandcontestscurrentlypopulargalaxyformationmodels.

Subjectheadings:halo—Galaxy:structure—Galaxy:LocalGroup—galaxies:kinematicsanddynamics—galaxies

An all high-latitude sky survey for cool carbon giant stars in the Galactic halo has revealed 75 such stars, of which the majority are new detections. Of these, more than half are clustered on a Great Circle on the sky which intersects the center of Sagitt

1.Introduction

Oneofthemostintriguingpuzzlesofmod-ernastronomyistheapparentexistenceofunseenmassongalacticandlargerscales.EvenfortheMilkyWay,ourknowledgeofthedarkcomponentispoor,thelittleknownhavingbeengleamedfromindirect,mostlykinematic,probes.

ThebestconstraintsonthemassdistributionintheGalaxyhavebeenderivedfromthekinematicsoftracersoftheGalacticdisk,inparticulartheHIgas,whichsamplestheGalaxyfromalmostitscen-tertotheoutermostpartsofitsdisk.Indeed,thesuccessofthistechniquein ttingtheHIveloc-itiesatGalacticlongitudes >30 stronglysug-gests(Dehnen&Binney1998)thattheGalaxyisaxisymmetricatGalactocentricdistancesgreaterthan～5kpc—http://rmationontheverticalmassgradienthasbeenobtainedfromthestudyoftracersoftheGalac-ticspheroid(Wyse&Gilmore1986;Binney,May&Ostriker1987;MorrisonFlynn&Freeman1990;Sommer-Larsen&Zhen1990;vanderMarel1991;Amendt&Cuddleford1994;Morrisonetal.2000).Yetdespiteconsiderableobservationalandtheo-reticale ort,theverticalstructureofthespheroidcomponentisstillpoorlyconstrained;dependentonthetechniqueemployed,thespheroid’soblate-nessisfoundtolieintherange0.5<q<1.(ItisusuallyassumedthatthedensityofthespheroidalcomponentsoftheGalaxyisoftheformρ(s),wheres=(R2+(z/q)2)1/2,andR,zareGalac-ticcylindricalcoordinates).Thispaucityofinfor-mationontheverticalstructureoftheGalaxy,ittranspires,istheprimarycauseofthelargeuncer-taintiesontheGalacticmassmodels(Dehnen&Binney1998).

Theabovestudiesofthestellarspheroiddonot,however,necessarilyplacestrongconstraints

An all high-latitude sky survey for cool carbon giant stars in the Galactic halo has revealed 75 such stars, of which the majority are new detections. Of these, more than half are clustered on a Great Circle on the sky which intersects the center of Sagitt

2.TheHalocarbonstarsample

TheAPMHalocarbonstarsurveyreportedinTotten&Irwin(1998),Totten,Irwin&White-lock(2000)andIrwin&Totten(2000)isnowalmostcomplete.PalomarskysurveyplatesofthewholenorthernskyatGalacticlatitudes|>30 ,aswellasUKSchmidtTelescopeplatescoveringb|～

mostofthesouthernskyatlatitudes|>30 (seeFigure1)haveanalyzedb|～

beenscannedandtosearchforgiantcarbonstars(C-stars).Theseintermediate-ageasymptoticgiantbranchstarsareintrinsicallyluminous,rare(≈1per200sq.deg)andveryredinopticalpassbands(Totten&Irwin1998),andcanthereforeeasilybedistinguishedfromotherstellarspecies, …… 此处隐藏：48063字，全部文档内容请下载后查看。喜欢就下载吧 ……